This invention relates to an angular position detector suitable for use in an internal combustion engine control system.
It is already known to employ a toothed wheel on the engine crankshaft with a fixed sensor which provides a pulse train as the wheel rotates, the pulse train being used to provide information about both the speed and angular position of the crankshaft. It is, however, necessary, when measuring the angular position to provide a signal at a specific datum position so that the position of the crankshaft can be measured from that datum position. GB-A No. 2065310 discloses the idea of omitting one of the teeth. The time intervals between the pulses are measured and when a time interval more than 1.5 times longer than the previous one is detected it is assumed that the "missing tooth" is passing the sensor and the next arriving pulse is treated as defining the datum position.
It is desirable for accurate engine timing control to ensure that the datum position is close to the top dead centre position in respect to one of the cylinders of the engine. Accordingly, it is proposed in GB-A No. 2065310, to put the "missing tooth" at this top dead centre position, the datum position then being, say, 10.degree. behind this top dead centre position.
With such an arrangement, however, problems can arise during engine starting, particularly in very cold conditions. In such conditions the load on the starter motor during each compression stroke can be such as to reduce the instantaneous cranking speed sufficiently to make an inter-pulse interval (other than that occurring at top dead centre) 50% longer than the previous interval, due to the reduced cranking speed so that a false datum position signal is produced where there is no gap detected, due to a false detection of the usual spacing between adjacent teeth or the gap.